By government regulation, steel pipe used in buried pipelines must be provided with a protective coating. The water-proof coating provides corrosion protection and usually some degree of electrical insulation. A variety of coating materials are used, including polyethylene or other polyolefin-based coatings. Polyethylene and similar polyolefin-based pipe coatings are applied to the outer surface of steel pipe in a three step process. A thin layer of an epoxy-based coating is first applied, with excellent binding properties to the bare metal of the pipe. An adhesive coating, usually comprising a functionalized polyolefin, is applied over the epoxy and, finally, the polyethylene or other polyolefin-based outer coating of the pipe is applied over the adhesive coating. Once the coatings are set, the polyolefin outer coating is bonded securely to the adhesive intermediate layer, while the adhesive intermediate layer is bonded securely both to the polyethylene and to the epoxy layer, which in turn is bonded securely to the pipe. The result is a durable, and relatively inexpensive, polyolefin coating over the outer surface of the pipe, bonded securely to the pipe, through the intermediate layers, without substantial risk of delamination.
When lengths of polyolefin-coated pipe are joined by welding to form a pipeline or similar structure, it is necessary to have bare metal exposed in an area adjacent to the weld in order to form, clean and inspect the welded joint. Also, heat from the welding operation would tend to damage immediately adjacent polymer coating materials. Accordingly, it is customary to leave the pipe bare of coating material for a distance on each end. When two such lengths are joined by a weld, the length of bare metal pipe exposed on each side of the welded joint varies depending upon the diameter of the pipe and the coating material, but a distance of at least 6 inches from the end of each pipe section is typical. This leaves an area of at least 12 inches of bare metal pipe for each welded joint, which also provides room for a grinder to be applied to the pipe to clean the weld after it is completed.
The desired length of bare metal at each end of each length of coated pipe is left uncoated at the factory when the pipe coating initially is applied. Once the weld is completed and inspected, there is a length of uncoated steel pipe which must be provided with a suitable watertight corrosion-inhibiting protective coating prior to burying or other completion of the pipeline.
While it would be possible to apply a polyolefin coating in the weld area, following the same three layer coating process used for producing the original polyolefin coating, this process is difficult and expensive to utilize under field conditions. Accordingly, it is the primary object of the present invention to provide a method and apparatus for protecting the weld area of polyolefin coated pipe.
Another object is to provide a protective coating for the weld area of polyolefin coated pipe which may be applied easily, reliably and inexpensively under field conditions.
A further object is to provide such a protective coating for the weld area of polyolefin-coated pipe which, when applied, provides the same or substantially the same corrosion inhibiting and electrically insulating characteristics to the coated pipe in the weld area as are provided for the pipe body by the polyolefin coating.
A still further object is to provide such a protective coating for the weld area of polyolefin-coated pipe in which the weld area protective coating adheres well to, and does not tend to delaminate from, both the bare metal of the pipe in the weld area and the materials of the polyolefin coating on the body of the pipe.